Period-Luminosity Relation for Type II Cepheids

TL;DR

This study establishes the period-luminosity relation for Type II Cepheids in the LMC using infrared and optical data, confirming its reliability as a distance indicator across different systems.

Contribution

The paper derives and calibrates the PLR for Type II Cepheids in the LMC, confirming its universality and establishing it as a new distance measurement tool.

Findings

Infrared PLRs are consistent with previous globular cluster results.

LMC distance modulus estimated at 18.46 mag from pulsation parallaxes.

RV Tau stars in the LMC differ from those in globular clusters.

Abstract

We have estimated JHKs magnitudes corrected to mean intensity for LMC type II Cepheids found in the OGLE-III survey. Period-luminosity relations (PLRs) are derived in JHKs as well as in a reddening-free VI parameter. The BL Her stars (P<4d) and the W Vir stars (P=4 to 20d) are co-linear in these PLRs. The slopes of the infrared relations agree with those found previously for type II Cepheids in globular clusters within the uncertainties. Using the pulsation parallaxes of V553 Cen and SW Tau, the data lead to an LMC modulus of 18.46+-0.10 mag, uncorrected for any metallicity effects. We have now established the PLR of type II Cepheids as a distance indicator by confirming that (almost) the same PLR satisfies the distributions in the PL diagram of type II Cepheids in (at least) two different systems, i.e. the LMC and Galactic globular clusters, and by calibrating the zero point of the PLR. RV Tau stars in the LMC, as a group, are not co-linear with the shorter-period type II Cepheids in the infrared PLRs in marked contrast to such stars in globular clusters. We note differences in period distribution and infrared colors for RV Tau stars in the LMC, globular clusters and Galactic field. We also compare the PLR of type II Cepheids with that of classical Cepheids.